This invention relates generally to microwave processing of high performance polymers, composites and the sintering of ceramics, and more specifically to the design of molds for processing of such materials including the selection or modification of materials to form the mold to provide for uniform heating of polymers, composites or ceramics by microwave energy.
Efforts to use radio frequency (RF) or microwave processing of polymers and composites has been pursued over the past few decades and yields substantial advantages. In contrast to conventional thermal treatments, the advantages of RF and microwave processing include rapid volumetric heating, avoidance of overheating at the surface, reduced processing time and reduced degradation of the processed polymers. RF dielectric heating may be efficient in applications where uniform volumetric heating is required for workpieces of large volumes and dimensions. RF heating has found a wide range of applications including the drying of wood, the gluing of wood and plastic products, and plastic sealing. Such applications usually employ relatively low radio frequencies (f), i.e. f=13.56 or 27.12 MHz, which are permitted for commercial use. Many applications for RF heating are found in the automobile industry. For example, RF bonding techniques have been used successfully for gluing of two parts of the back door of the ZX and XANTIA model automobiles. In my U.S. Pat. No. 6,241,929 (hereafter the “929” Patent) I disclosed a method for molding objects of complex shape using RF heating. Known techniques for RF dielectric heating are characterized by the presence of two or more metal electrodes electrically separated from each other across which an RF electrical field is applied from a generator, with the working material being placed between the metal electrodes.
In most cases, the efficiency of dielectric heating increases with an increase in the frequency of the electromagnetic field. The preferred frequency whose use is permitted is the frequency allocated for commercial use of microwave, i.e. f=2450±50 MHz. At elevated frequencies of the electromagnetic field, the same heat rating for a particular material may be achieved with much lower strengths of electric field in comparison with lower frequencies. For example, the field strength E will be reduced approximately 10 times at the frequency f=2450 MHz in comparison with that at f=27.12 MHz providing same power rating (see formula (1)). At considerably reduced field strengths, the problem of arcing, which is very common for radio frequencies, will be eliminated completely.
At such high microwave frequencies, the design of the applicator for heating the work material is significantly different from an RF applicator with metal plates and conventional open wire circuits. Microwave processing is generally performed within a metallic microwave applicator, which may be a traveling wave applicator, resonant single mode or multimode applicators. The size of such applicators usually exceeds several wavelengths at a given frequency. By definition, microwave frequencies range from 300 MHz to 300 GHz and therefore the applicator size may be about 1 meter or more. The single mode and traveling wave applicators may be used in processing simple material shapes such as fibers. However, the multimode applicator has the capability of coupling microwave energy onto materials of large and complex shape.
The multimode applicator generally comprises a closed metal box having a cavity or chamber and some means of coupling to a power source or generator such as waveguides or antennas. The dimensions of the cavity should be several wavelengths long in at least two dimensions. Such a box will support a large number of resonant modes in the applicator cavity in a given frequency range. When the applicator is not loaded, each of these modes is characterized by a sharp resonance at a given frequency. It is important to provide as many of these modes as possible to lie near the operating frequency of microwave generator (or generators). When such an applicator is loaded with a microwave absorbing work material, the resonance curves will overlap to give a continues coupling into the load. The overall distribution of electromagnetic energy is not uniform throughout the microwave cavity or the work material resulting in high and low energy field areas. Such hot and cold spots can be observed in household microwave ovens and are tolerated for food applications, because relatively high thermal conductivity of water containing food results in reductions of the thermal variations established due to non-uniform heating. But this is not the case for high performance polymers since most of these polymers exhibit very poor thermal conductivity. Any attempt to heat the polymer work material in a conventional microwave oven without specially designed molds will lead to overheating or burns of the polymer in some places while in other its places will be under heated or cured. Uniformity of heating is therefore of great importance in the case of polymer processing.
There have been numerous attempts in the prior art to achieve uniform microwave fields in the volume of a workpiece to be heated. Examples of such techniques include multiple slot entry techniques or the development of “stirred” multimode cavities, in which the field is constantly scanning in order to average out hot and cold spots. While these methods provide some improvement, it has not been possible to achieve desired uniformity of temperature field in the work material. Better uniformity of field can be obtained at a frequency of 2450 MHz by substantially increasing the cavity dimensions (approximately 100 wavelengths) which will require a very large microwave power supply to produce sufficient energy density within the cavity of 12 meters size. A more feasible way is to employ higher frequencies, as high as 28 GHz, where 100 times wavelength is approximately 1 meter in size. However, operation of microwave ovens at a frequency of 28 GHz is considered too expensive and is out of the permitted frequency range.
Another known technique is the excitation of multiple standing-wave modes in the microwave cavity by a plurality of magnetrons. For example, a commercial microwave oven designed for the food industry, such as Panasonic model NE-3280, has 3.2 kW of microwave power and is powered by four magnetrons. Uniformity of heating is significantly improved using such a microwave oven. Polymer processing in such microwave ovens will require specially designed molds for each particular polymer or polymer groups.
Recently developed variable frequency microwave (VFM) ovens may offer an advantage in polymer processing. The advantage of VFM processing over conventional fixed frequency microwave processing is its ability to provide uniform heating over a large volume of a work material (the material to be molded). With VFM heating, a large number of frequencies are introduced into the cavity sequentially during sweeping of frequency in a wide frequency range. Each incident frequency establishes a different pattern of heating. When a sufficient bandwidth is used, time-average uniform heating can be achieved with proper adjustment of the frequency sweep rate and sweep range. A disadvantage of such a technique is that presently the maximum microwave power available does not exceed 500 W, and VFM ovens are not generally commercially available. The price of such ovens is expected to be very high in comparison with fixed frequency microwave ovens. Also, VFM ovens operate in the range of frequencies not permitted for commercial use.
A method for the uniform heating of a workpiece or work material in a microwave oven operating at a frequency of 2450 MHz is disclosed in U.S. Pat. No. 5,202,541. The workpiece assembly represents a multilayer structure of ceramic components placed in a powder bed and surrounded by metal rings stacked vertically in the direction of electric field. The metal rings are electrically separated from each other. The number, dimensions and separation of employed rings in any particular case is determined by trial and experimentation to achieve the desired uniform electrical field. Alternatively, the rings may be placed snugly against one another to create a conductive wall along the electric force lines and surrounding the crucible containing workpiece assembly. It is noted that depending on the dimensions and nature of the load assembly, the location and extent of the various hot and cold regions can vary.
Experimentally it has been shown that regions with undesirably wide variations of temperature arise in the load whenever an attempt is made to increase the size of workload assembly and quantity of heating ceramic components. In the description of U.S. Pat. No. 5,202,541 there is no analysis of the heat exchange between the workpieces, the powder bed, the crucible and the metal rings and how the difference of their dielectric, thermal and mechanical properties will affect the uniformity of heating. It is evident that workpieces may be heated uniformly only if temperature rise ratings of workpieces and surrounding medium are equal. At this condition zero heat exchange between different components will allow the formation of a uniform temperature field over entire volume of assembly. As it is shown hereafter the matching of parameter tanδ/εcρ of the work material and surrounding media adjacent to workpiece is necessary to provide for uniform heating of the whole assembly.
Another drawback of this method relates to the working condition of the microwave generator. The presence of tall conducting metal rings having a total height comparable with the dimension of microwave cavity may cause significant reflection of microwave energy toward the generator and may affect its safety during operation.
U.S. Pat. No. 4,307,277 discloses a method for microwave heating of a ceramic work-piece. A work piece is surrounded by an inner casing, which is made of microwave absorbing material. An intermediate casing made of refractory insulator covers inner casing for thermal insulation purpose. The whole assembly is placed inside a conventional household microwave oven and exposed to microwave radiation. The work material does not absorb microwave energy and is heated by heat radiation from the inner layer. Such and apparatus may provide uniform heating only of a small volume work-piece since heat flows from its surface to the center due to thermal conductivity. Such a method cannot be used for uniform heating of thick or massive work-pieces and will face the same challenges as conventional methods employing infrared radiation.
U.S. Pat. No. 4,617,439 describes a method for uniform heating of a relatively thin planar panel of work material placed between two metal plates and in intimate contact therewith. Such a sandwich structure of metal plates and work material may be stacked in the direction of the electric field. It has been stated that the effect of electric field guiding by the metal plates allows for the control of the distribution of energy within the work material to be heated. The plurality of stacked panels with metal plates therebetween provides an even distribution of energy in all of the panels. In such a method, the metal panels may have different contours to shape the layers of work material into the desired profile by means of compression and heat. During exposure of the assembly to microwave radiation, the metal panels will not be heated by the microwave energy due to the skin-effect. The purpose of these metal plates is to guide the electric field and equalize the temperature between hot and cold spots in the work material due to their high thermal conductivity. Actually, these plates work as high thermal conductivity heat absorbers. On one hand, the intimate contact of the metal plates with the work material will equalize the temperature in the longitudinal direction between hot and cold spots, but on the other hand, it will cause the flow of heat from the heated work material to the cold plates resulting in radiation of heat from their surfaces and temperature gradients in the direction normal to their interface. To avoid this disadvantage in the present invention, metal plates with high thermal conductivity are placed between the inner mold layer and the work material. With the proper selection of material to form the mold, thermal gradients on the interface between the mold and the work material may be significantly reduced in both and longitudinal and normal directions.